1. Field of the Invention
This invention relates to a method for the production of a semiconductor device, and more particularly, it relates to a method for the production of a compound semiconductor device which requires accurate control in the depth of impurity ions implanted into a semiconductor layer.
2. Description of the Prior Art
With a recent development in compound semiconductor devices, there is an increasing demand for the implantation of p-type impurity ions into a compound semiconductor for the purpose of producing a semiconductor device with improved characteristics at a lower cost.
As an example of compound semiconductor devices, a heterojunction bipolar transistor (hereinafter abbreviated as an "HBT") is shown in FIG. 2. A conventional process for producing this HBT will be described below.
First, on an n.sup.+ -GaAs substrate 4, an n.sup.- -GaAs collector layer 3 is formed. Then, p-type impurity ions are implanted into the n.sup.- -GaAs collector layer 3, followed by annealing for activation, resulting in a p-type base layer 8. On the p-type base layer 8, an n-AlGaAs emitter layer 5 is grown. Thereafter, n-sided contact electrodes 7 are formed from AuGe/Ni/Au respectively on the upper face of the n-AlGaAs emitter layer 5 and on the back face of the n.sup.+ -GaAs substrate 4. The contact electrode 7 on the n-AlGaAs emitter layer 5 functions as an emitter electrode, while the contact electrode 7 on the n.sup.+ -GaAs substrate 4 functions as a collector electrode.
Then, part of the n-AlGaAs emitter layer 5 and n-sided contact electrode 7 formed thereon is removed by mesa etching in an etchant containing phosphoric acid, thereby exposing the corresponding part of the p-type base layer 8. On the exposed surface of the p-type base layer 8, p-sided contact electrodes 6 are formed from AuZn/Au as base electrodes, resulting in an HBT shown in FIG. 2.
The base width (the thickness of the base layer 8) in the thus produced HBT is one of the important parameters which determine the characteristics of the HBT. In order to accurately control the base width, the impurity ions implanted into the n.sup.- -GaAs collector layer 3 are required to have a sharp and accurate distribution even after the activation annealing and other heat treatments for further crystal growth.
Beryllium (Be) and zinc (Zn) are known as p-type impurities used in ion implantation for the formation of a base layer. However, neither of them are suitable for practical use for the following reasons. Be is toxic and has a large diffusion coefficient in a compound semiconductor. Zn also has a large diffusion coefficient when implanted into a compound semiconductor layer, and is likely to be removed away from the surface of the compound semiconductor layer during the subsequent activation annealing. Therefore, the use of carbon ions for ion implantation has been proposed because carbon has no toxicity and has a small diffusion coefficient when implanted into a compound semiconductor (see, e.g., S. Yamahata, Shingaku-Giho Electronic Devices, ED89-56 (1989)).
Alternatively, an ion implantation method has been developed where SiF.sup.+ or SiF.sub.2.sup.+ ions are implanted in a compound semiconductor layer (see, e.g., Japanese Laid-Open Patent Publication No. 63-244842).
In order to produce a semiconductor device with excellent characteristics, the thickness of each layer in the device should be made small. However, it is impossible to form a thin doped layer using the implantation of carbon ions, as will be described below.
When carbon ions are introduced as an impurity into a semiconductor layer to form a doped layer therein, the projected range (i.e., Rp) of the implanted carbon ions is large because carbon has a small atomic weight. Such a large projected range makes it difficult to obtain a shallow and sharp distribution of the implanted carbon ions at an acceleration voltage of a practical level. Furthermore, the large projected range causes a great possibility that channeling will arise during the ion implantation. Therefore, the thickness of the doped layer to be formed by the implantation of carbon ions cannot be stably controlled. The large projected range of the implanted carbon ions has prevented the implantation of carbon ions from being put into practical use for the formation of a thin doped layer.